Microcapsules are small particles of solids or droplets of liquids inside a thin coating of a shell material such as beeswax, starch, gelatin, or polyacrylic acid. They are used, for example, to prepare liquids as free-flowing powders or compressed solids, to separate reactive materials, to reduce toxicity, to protect against oxidation and/or to control the rate of release of a substance such as an enzyme, a flavor, a nutrient, a drug, etc.
In the past, research has concentrated on so-called “single-core” microcapsules. However, one of the problems with single-core microcapsules is their susceptibility to rupture. Thus, others have tried to increase the thickness of the microcapsule wall in order to increase the strength and/or impermeability of such microcapsules. However, this practice can lead to a reduction in the loading capacity of the microcapsule.
Another approach to improve microcapsules has been to create so-called “multi-core” microcapsules. For example, U.S. Pat. No. 5,780,056 discloses a “multi-core” microcapsule having gelatin as a shell material. These microcapsules are formed by spray cooling an aqueous emulsion of oil or carotenoid particles such that the gelatin hardens around “cores” of the oil or carotenoid particles. Yoshida et al. (Chemical Abstract 1990:140735 or Japanese Patent Publication JP 01-148338) discloses a complex coacervation process for the manufacture of microcapsules in which an emulsion of gelatin and paraffin wax is added to an arabic rubber solution and then mixed with a surfactant to form “multi-core” microcapsules. Ijichi et al. (J. Chem. Eng. Jpn. (1997) 30(5):793-798) microencapsulated large droplets of biphenyl using a complex coacervation process to form multi-layered microcapsules. U.S. Pat. Nos. 4,219,439 and 4,222,891 disclose “multi-nucleus” oil-containing microcapsules having an average diameter of 3-20 μm with an oil droplet size of 1-10 μm for use in pressure-sensitive copying papers and heat sensitive recording papers. While some improvement in the strength of microcapsules may be realized by using methods such as these, there remains a need for microcapsules having improved impermeability and good oxidative barrier to the encapsulated substance, preferably in conjunction with high load volumes. Disclosed herein are compositions and methods which meet these and other needs.